Method for the manufacture of a coated steel sheet

Abstract

A steel sheet coated with a coating comprising from 10 to 40% of nickel, the balance being zinc, such steel sheet having a microstructure comprising from 1 to 50% of residual austenite, from 1 to 60% of martensite and optionally at least one element chosen from: bainite, ferrite, cementite and pearlite, and the following chemical composition in weight: 0.10<C<0.50%, 1.0<Mn<5.0%, 0.7<Si<3.0%, 0.05<Al<1.0%, 0.75<(Si+Al)<3.0% and on a purely optional basis, one or more elements such as Nb≤0.5%, B≤0.005%, Cr≤1.0%, Mo≤0.50%, Ni≤1.0%, Ti≤0.5%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration.

What is claimed is: 
     
       1. A steel sheet comprising:
 a steel substrate; and 
 a coating on the steel substrate, the coating comprising from 10 to 40% of nickel, a balance being zinc, 
 the steel substrate having a microstructure comprising from 1 to 50% of residual austenite, from 1 to 60% of martensite, from 10 to 40% of bainite, such bainite comprising from 10 to 20% of lower bainite, from 0 to 15% of upper bainite and from 0 to 5% of carbide free bainite, and optionally at least one element chosen from: ferrite, cementite and pearlite, and the following chemical composition in weight:
   0.10<C<0.50%, 
   1.0<Mn<5.0%, 
   0.7<Si<3.0%, 
   0.05<Al<1.0%, 
   0.75<(Si+Al)<3.0%, 
 
 and on a purely optional basis, one or more elements:
   Nb≤0.5%,
 
   B≤0.005%,
 
   Cr≤1.0%,
 
   Mo≤0.50%,
 
   Ni≤1.0%,
 
   Ti≤0.5%,
 
 
 
       a remainder of the composition making up of iron and inevitable impurities resulting from processing. 
     
     
       2. The steel sheet as recited in  claim 1  wherein the coating includes from 10 to 30% by weight of nickel. 
     
     
       3. The steel sheet as recited in  claim 2  wherein the coating includes from 10 to 20% by weight of nickel. 
     
     
       4. The steel sheet as recited in  claim 3  wherein the coating consists of zinc and nickel. 
     
     
       5. The steel sheet as recited in  claim 1  wherein the coating is directly in contact with the steel sheet. 
     
     
       6. The steel sheet as recited in  claim 1  wherein the coating has a thickness between 5 to 15 μm. 
     
     
       7. The steel sheet as recited in  claim 6  wherein the coating has a thickness between 5 to 10 μm. 
     
     
       8. The steel sheet as recited in  claim 1  wherein the steel sheet microstructure includes from 5 to 25% of residual austenite. 
     
     
       9. The steel sheet as recited in  claim 1  wherein the 1 to 60% of martensite includes from 1 to 60% of tempered martensite. 
     
     
       10. The steel sheet as recited in  claim 1  wherein the steel sheet microstructure includes from 1 to 25% of ferrite. 
     
     
       11. The steel sheet as recited in  claim 1  wherein the 1 to 60% of martensite includes from 1 to 15% of untempered martensite. 
     
     
       12. A spot welded joint of at least two metal sheets comprising: the steel sheet as recited in  claim 1  and a second metal sheet, the joint containing less than 2 cracks having a size above 100 μm and wherein the longest crack has a length below 250 μm. 
     
     
       13. The spot welded joint as recited in  claim 12  wherein the second metal sheet is a steel sheet or an aluminum sheet. 
     
     
       14. The spot welded joint as recited in  claim 12  wherein the second metal sheet includes a second steel substrate; and a second coating on the second steel substrate, the second coating comprising from 10 to 40% of nickel, a balance being zinc, the second steel substrate having a microstructure comprising from 1 to 50% of residual austenite, from 1 to 60% of martensite and optionally at least one element chosen from: bainite, ferrite, cementite and pearlite, and the following chemical composition in weight:
   0.10<C<0.50%, 
   1.0<Mn<5.0%, 
   0.7<Si<3.0%, 
   0.05<Al<1.0%, 
   0.75<(Si+Al)<3.0%, 
 and on a purely optional basis, one or more elements:
   Nb≤0.5%,
 
   B≤0.005%,
 
   Cr≤1.0%,
 
   Mo≤0.50%,
 
   Ni≤1.0%,
 
   Ti≤0.5%,
 
 
 a remainder of the composition making up of iron and inevitable impurities resulting from processing. 
 
     
     
       15. The spot welded joint as recited in  claim 12  further comprising a third metal sheet being a steel sheet or an aluminum sheet. 
     
     
       16. The spot welded joint as recited in  claim 12  wherein the joint contains no cracks having a size above 100 μm. 
     
     
       17. An automobile vehicle part comprising the steel sheet as recited in  claim 1 . 
     
     
       18. A method for the manufacture of a coated steel sheet according to  claim 1 , comprising the following steps:
 providing an annealed steel substrate having the following chemical composition in weight:
   0.10<C<0.50%, 
   1.0<Mn<5.0%, 
   0.7<Si<3.0%, 
   0.05<Al<1.0%, 
   0.75<(Si+Al)<3.0%, 
 
 and on a purely optional basis, one or more elements:
   Nb≤0.5%,
 
   B≤0.005%,
 
   Cr≤1.0%,
 
   Mo≤0.50%,
 
   Ni≤1.0%,
 
   Ti≤0.5%,
 
 
 such steel sheet being annealed at a temperature between 600 to 1200° C.; 
 the steel substrate having a microstructure comprising from 1 to 50% of residual austenite, from 1 to 60% of martensite, from 10 to 40% of bainite, such bainite comprising from 10 to 20% of lower bainite, from 0 to 15% of upper bainite and from 0 to 5% of carbide free bainite, and optionally at least one element chosen from: ferrite, cementite and pearlite; and 
 coating the annealed steel substrate with a coating comprising from 1 to 40% of nickel, a balance being zinc. 
 
     
     
       19. A spot welded joint of at least two metal sheets comprising: the coated steel sheet obtained from the method as recited in  claim 18  and a second metal sheet, the joint containing less than 2 cracks having a size above 100 μm and wherein the longest crack has a length below 250 μm. 
     
     
       20. The method as recited in  claim 18  wherein the steel substrate is annealed in a continuous annealing. 
     
     
       21. The method as recited in  claim 18  wherein the annealing is performed in an atmosphere comprising from 1 to 30% of H 2  at a dew point between −10 and −60° C. 
     
     
       22. The method as recited in  claim 18  wherein the coating is deposited by vacuum deposition or an electro-plating method. 
     
     
       23. The method as recited in  claim 22  wherein the coating is deposited by the electro-plating method. 
     
     
       24. An automobile vehicle part comprising a coated steel sheet obtained by the method as recited in  claim 18 . 
     
     
       25. The steel sheet as recited in  claim 1  wherein the steel sheet microstructure comprises from 5 to 25% of residual austenite and from 1 to 25% of ferrite.